Anatomy of the Human Eye

Anatomy of the Human Eye
Retina
• Despite its peripheral location is part of the CNS.
• Converts light energy into action potentials that travel out
the optic nerve into the brain.
• Is layered, relatively simple for a CNS structure.
• Surrounded on one side by pigmented epithelium. Contains
melanin that helps reduce backscattering of light. Also
plays a role in maintenance of photoreceptors.
• 5 types of neurons in the retina: photoreceptors, bipolar
cells, ganglion cells, horizontal cells and amacrine cells.
• A direct 3 neuron chain is the basic unit of transmission.
Photoreceptor (rods and cones) to bipolar cell to
ganglion cell (axons form the Optic nerve)
Structural Differences Between Rods and Cones
rods and cones are distinguished by:
• shape
• type of photopigment they contain
• distribution across the retina
• pattern of synaptic connections
• specialized for different aspects of vision
Rod system-low spatial resolution but extremely
sensitive to light
Cone system- high spatial resolution but is relatively
insensitive to light.
Phototransduction
• Photoreceptors do not exhibit action potentialslight causes a graded change in membrane
potential that changes the rate at which
neurotransmitter is released.
– The NT is thought to be glutamate
• Light absorption leads to hyperpolarization of
the neuron. This leads to less release of
neurotransmitter to post-synaptic cell.
Cones and rods hyperpolarize in response to light
What does light do?
• In the dark, resting potential of the photoreceptor
is - 40 mv.
• Light shining onto outer segment leads to the
hyperpolarization of the photoreceptor and
reduction of neurotransmitter released.
• In the dark the number of Ca++ channels open at
the synaptic terminal is high, and rate of
neurotransmitter release is high.
• In the light number of open channels is reduced
and rate of neurotransmitter release is reduced.
cGMP gated Na+ channels are key
in dark channel open due
to cGMP binding.
Na+ rushes in
cell depolarized
exposure to light causes cGMP
breakdown, closing Na+ permeable
channels, and hyperpolarization
Light transduction
• Light is detected by
retinal
Rhodopsin (from rods) is
a combination of retinal,
the light-absorbing
molecule, and a large
protein called opsin. The
opsins are closely related
in structure to the Gprotein-coupled
receptors.
Details of Phototransduction in Rod Photoreceptors
http://webvision.med.utah.edu/photo1.html
Rods vs. Cones
• Rods produce a reliable response to a single photon
– it takes over a 100 photons to produce a comparable
response in a cone.
• Cones adapt faster than rods
– about 200ms for a cone; 800ms for a rod.
• Rods synapse onto specific bipolar cells (rod bipolars) that
synapse onto amacrine cells which contact both cone
bipolars and ganglion cells.
– Cones go to bipolar cell to RGC directly.
• Rods exhibit convergence-many rods synapse onto
(converge on) a single bipolar cell, many bipolars onto a
single amacrine cell.
– cones can be 1-1-1
Rods and cones are not distributed
equally in the retina
• Human - 91 million rods, 4.5 million cones.
• In most places the density of rods exceeds that of cones.
• Cones increase in density 200 fold in the fovea, become
highly packed. Center of the fovea is rod free.
• Gives high visual acuity, which decreases rapidly away
from the fovea.
• Reason why we constantly move our eyes toward what we
want to look at.
– And why it it best to see a dim object by looking away from it.
Other cell layers are displaced in the fovea. Light
hits cones with less interference.
cones and color vision
• 3 types of cone, each having different absorption
spectra- called blue, green, and red opsin.
• Most people can match any color by changing the
intensities of these three colors (RGB).
• 5-6% of males are color blind- due to mutations in
red and green opsins. The pigment genes are Xlinked and near each other.
Rods and Cones
• Rods
– 90 – 120 million
– Peripheral vision
– Located everywhere
except fovea
– Very sensitive to light
– Used in low light
situations
– One type
– Highly convergent
– Black and White
• Cones
– 4-6 million
– Central vision
– High density in the
macula and fovea
– Less sensitive to light
– Most normal lighting
conditions
– Three types (red, green,
blue)
– Nonconvergent
– Color vision
Retinal ganglion cells (RGC)
• Record from an RGC and shine light onto different
photoreceptors. Find:
• Even in the dark RGCs are spontaneously active.
• Receptive fields of RGCs are circular. Smaller in the
center and bigger in the periphery
• The receptive fields of RGCs overlap so that multiple
RGCs see each point of space.
• http://webvision.med.utah.edu/IPL.html
Responses of On- and Off-Center Retinal Ganglion Cells to
Stimulation of Different Regions of their Receptive Fields
Summary- Retinal GanglionCells
• For an on-center RGC, a point of light that just fills the
center will give maximal stimulation (increased action
potentials).
• Light that crosses into surround will yield intermediate
response depending on the relative amounts.
• Both center and surround illuminated is similar to being in
the dark (background levels).
• Ganglion cells fire depending on contrast, not by absolute
light intensity.
Light in center -> on ganglion cells increase firing rate,
off ganglion cells decrease firing rate
glutamate
mGluR6
glutamate
AMPA
Kainate
NMDA
AMPA
Kainate
Antagonistic Surrounds of Retinal Ganglion Cell Receptive Fields
light hits surround cones
- horizontal cell (surround) are
hyperpolarized and release less
GABA onto cone
in center, depolarizes center cone.
More glutamate released onto
Bipolars from center cone.
Off-center depolarized, on-center
Hyperpolarized by glutamate from
center cone.
Off-center ganglion cell fires more
On-center fires less.
GABA
glutamate
mGluR6 AMPA
Kainate
glutamate
AMPA
Kainate
NMDA
Subtypes of RGCs
• 2 Types:each has on and off center
– M (Magnocellular)
• 10%
• Large receptive fields
• Detection of movement
– P (Parvocellular)
• 90%
• Center surround cells for color
– Red-Green
– Blue-Yellow
• Detection of form and fine detail
Summary
• Light falls on photopigment, that is transformed to
action potentials that ganglion cells convey to the
brain.
• Phototransduction occurs in rods and cones that
have different properties that meet the conflicting
demands of sensitivity and acuity.
• RGCs have a center-surround arrangement of
receptive fields that makes them good at contrast
detection and relatively insensitive to background
illumination.
Central Visual Pathways:
Retinal Targets
• The retina (axons of RGCs) projects to multiple areas.
Each area is specialized for different functions.
– Lateral geniculate nucleus (LGN)- in the thalamusreceives input from retina and sends it to visual cortex.
Most important visual projection for perception.
– Pretectum-located at midbrain-thalamus boundary.
Responsible for pupillary light reflex.
– Superior colliculus-in midbrain, coordinates head and
eye movements.
– Suprachiasmatic nucleus- in hypothalamus-involved
in day/night cycles.
The human visual system: afferent projections
Circuitry Responsible for the Pupillary Light Reflex
Only one side is shown here
remember the projections
are symmetrical.
The spatial relationships among the
RGCs are maintained in their targets.
• Organized in maps.
• Images are inverted and left-right reversed as they are
projected onto the retina.
• The visual field can be divided into nasal, temporal,
inferior (ventral) superior (dorsal). Fixation point is where
the foveas align.
• The left half of the visual world is represented in the
right half of the brain and vice versa. (not left eye to
right side of brain, but left visual field)
Projection of the Visual Fields
onto the Retinas
• PN12041.JPG
Images are inverted on the retina
Projection of the Visual Fields
onto the Retinas
• PN12042.JPG
Projection of the Binocular Field
of View Relates to Crossing of
Fibers in Optic Chiasm
• PN12050.JPG
http://thalamus.wustl.edu/course/cenvis.html
The visual field
At the optic chiasm, visual
information from the two sides of
the head cross.
In animals with eyes on the sides of the
head, the entire visual field for each
side is sent to the opposite side (to the
tectum).
In forward-looking animals, the
visual image is split
An object on the right side of the visual
field is seen by both left hemi-retinas.
At the optic chiasm, the two left hemiretina projections go left, while the two
right hemi-retinas go right.
Fig 16-2
Visual field defects
• Because the spatial relationships in the
retina are maintained in the brain, a careful
analysis of the visual fields of a patient can
often indicate where brain damage is
located.
Visual Field Deficits Resulting from Damage Along the
Primary Visual Pathway
Black means blind
LGN
• 90% of retinal axons go to Lateral Geniculate
Nucleus (LGN).
• LGN projects to visual cortex (striate cortex).
• Contains 6 layers, specific to eye (ipsi vs contra)
and with type of ganglion cell, magnocellular
(gross shape and movement) or parvocellular
(form and color.)
• Layers align in order of visual fields.
Projection to LGN
Each LGN layer is eye-specific
The projections from the retinal ganglion
cells maintain the field of view as it was
seen - this is called a retinotopic map. The
LGN has 6 layers of cell bodies.
Each LGN receives input from both eyes,
but axons terminate in separate layers, so
LGN nuclei are monocular
Projection to cortex
The visual field is projected in a
retinotopic way.
The right visual field projects to
the left cortex; the left visual
field to the right.
The fovea, because of its high
sensitivity and density of cones,
is represented on one-fourth of
striate cortex.
Visuotopic Organization in the
Right Occipital Lobe
• PN12060.JPG
Course of Optic Radiations to Striate Cortex
Lower visual field (dorsal retina)
Upper visual field- ventral retina
Neurons in the primary visual cortex
respond selectively to oriented edges
• Hubel and Wiesel- measured responses of neurons
in visual cortex. Found that they respond to bars or
lines of specific orientations.
• Two types of cells:
– Simple: respond to stimulus only if matches orientation.
Responds to bars or lines, not to spots. They also have
surround inhibition. Receptive fields can be generated
by having 3-4 LGN neurons innervate one simple cell.
– Complex: bigger receptive fields, not strongly
orientation selective, no clear on or off zones, detect
movement.
Neurons in the Primary Visual Cortex
Respond Selectively to Oriented Edges
Neurons in the Primary Visual Cortex
Respond Selectively to Oriented Edges
• PN12092.JPG
Types of simple cell receptive fields
Complex cells – some are directionally sensitive,
other cells respond to length of stimulus
Info from multiple LGN cells of the
same orientation and field
converge on a singe simple cell.
Several simple cells
converge on a complex cells
red dots inhibitory synapses
Mixing of Pathways from the Two Eyes First Occurs in the
Striate Cortex: Layer IV projects to other layers and this is
where the info from both eyes converges on the same cells
• PN12100.JPG
Columnar Organization of Ocular Dominance
Aside form layer 4,
cells in the
superficial layers
respond with
varying degree to
input from right and
left eyes.
Electrode shows in
other layers the shift
in ocular dominance
is gradual. If the
electrode is vertical,
there is more strict
ocular dominance.
“hypercolumns” contain blobs which contain
color sensitive circular surround receptive fields
Columnar Organization of Orientation Selectivity in
the Monkey Striate Cortex
• PN12120.JPG
Vertical electrode – all cells stimulated by same orientation.
Across the layer, cells respond to different orientations.
Note in layer 4, the dots represent lack of orientation specific cells in this layer
Parallel processing in the visual system
• Separate pathways for color (Parvocellular) and
movement (Magnocellular).
• M and P ganglion cells in retina. M cells bigger,
larger receptive fields, faster conduction velocities,
and respond transiently to visual stimulation. P
cells smaller, respond in a sustained fashion.
• M and P go to different layers in the LGN and
project to different populations of cells in layer 4
of V1.
Magno- and Parvocellular Streams
Projection to cortex
The striate cortex is a six-layered structure - layer 4 is
the major input layer.
LGN inputs
Cortical cells
Stellate cells in layers 4 receive the input and project it to layers 2/3, which
then project to layers 5 and 6. 0utput of the cortex is via pyramidal cells in
layers 2, 5 and 6.
Extrastriate visual areas
• There are many other areas of the brain that process visual
information; each gets info from primary visual cortex
(V1).
• Specialized for different functions.
• Middle temporal area (MT), respond to direction of a
moving edge without regard to its color.
• V4 responds to color of a stimulus without regard to form.
• 10 different visual areas, each with a topographic map.
• Damage in these areas can give weird experiences.
Localization of Multiple Visual Areas in the Human
Brain Using fMRI
Organization of the Dorsal and
Ventral Visual Pathways
• PN12170.JPG
Weird visual defects
• Disruptions in V1 cause blindness, but some people can
“guess” what an object is. Implies that other projections
from eye to brain can somehow compensate.
• Cerebral achromatopsia- do not see in color-only black and
white. Legions in extrastriate cortex regions like V4 or in
ventral stream.
• Lesions in MT region cause defects in detecting motion.
Hard to pour drinks accurately.